Black water and Grey water tank issues

[abcan]

Hi we have purchased an Enduramax 30 MFW 2010 as a second-hand unit. It is in great condition other than the fact that we are having issues with the water tanks. We have been able to put 48 US gallons into the fresh water tank, which is good. The issue comes when we are actually using water in the unit. We found that the tub would not drain after a few days of camping. We have done some trouble shooting once we got it home. We drained all the tanks and then starting pouring water down the tub/shower in 6 US gallon increments. We were only able to pour 24 gallons down before it would no longer drain. We assumed that it was draining into the 'grey water' tank, however, when we checked the toilet drain the black tank was completely full. This means that the shower is draining into the 'sewer/black water' tank. The specs I found say the black water tank should be 54 gallons. We then decided to find out where the bathroom and kitchen sinks were draining into. We were able to pour approximately 32 gallons into the sink before it quit draining. Which would be fine except for the fact that the specs we have found say that the grey water tank should hold 54 gallons, and the gauge for grey water reads only 2/3 full. We believe that the possible reason it will not take anymore water is because the tank is not vented properly or the tank vent is plugged. We can find drain vents but no tank vent for the grey water. I really need some assistance in getting the proper specs for this unit that state what it was supposed to have when it left the factory. I am also looking for a service manual that contains the proper plumbing specs for it.